Anti-sweat heater control system and method

ABSTRACT

An improved anti-sweat heater control system is disclosed for reducing and/or eliminating sweat on cooled surfaces of a refrigerated container. A method for reducing or eliminating sweat from forming on cooled surfaces of a refrigerated container is also disclosed. A refrigerated container incorporating the anti-sweat heater control is also disclosed.

RELATED APPLICATIONS

This application claims provisional priority to U.S. Provisional Patent Application Ser. No. 60/559,113, filed 2 Apr. 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system and method for heating outer surfaces of a cooled container to prevent condensation or sweat.

More particularly, the present invention relates to a system and method for heating outer surfaces of a cooled container to prevent condensation or sweat, where the apparatus includes a container, a cooling system and a heating system, where the heating system maintains the outer surfaces above a dew point temperature of a lowest temperature point on the outer surfaces for which anti-sweat protection is desired.

2. Description of the Related Art

Many refrigerated display cases require electric strip heaters to be installed in the materials near the interface between the refrigerated zone and the ambient zone to prevent condensation (sweating) from occurring on the surfaces exposed to the ambient zone with it's potentially high levels of humidity. The strip heaters add enough heat to raise the exposed surfaces above the dewpoint of the highest design humidity condition. Years ago these heaters would be energized 100% of the time even if the ambient humidity is far less than the design humidity and the amount of heat required to prevent condensation is significantly less. Glass door coolers and freezers in particular require a large amount of “anti-sweat” heat.

The cost of electricity to operate these anti-sweat heaters can be significant. In addition to the direct cost of the electricity applied to the anti-sweat heaters, roughly half of the heat put into the frames ends up in the refrigerated space and adds a significant load to the compressor.

In the last 25 years, several control approaches have been made to reduce the amount of heat required to maintain frames sweat free. The basic approaches that have been used commercially are: (1) turn off heaters, if humidity is below a certain level (humidity sensing), (2) modulate the percentage oftime the heaters are energized based on a measured zone dewpoint (dewpoint sensing); and (3) detect the presence of moisture or the approach of condensation on an exposed surface and turn on the heat until the sensor no longer detects the need for heat (condensation sensing).

Although numerous sweat-free solutions to the problem of refrigerated display cases or for preventing condensation on outer surfaces of cooled containers have been tried and commercialized, none of these solutions provided an adequate and energy efficient protection against case or outer surface sweating. Thus, there is a need in the art for different system and method for sweat reduction and/or elimination on cold surfaces associated with cooled containers, especially, refrigerated display cases used in the grocery industry or related food industries or other cooled containers having outer transparent surfaces.

SUMMARY OF THE INVENTION

The present invention provides a system designed to minimize and/or eliminate condensation on desired outer surfaces of cooled containers, where the system includes a container having at least one outer surface for which anti-sweat protection is desired, a cooling system adapted to cool an interior of the container, a sensing subsystem for determining a temperature of at least one point on an outer surface of the container, a heating subsystem adapted to heat the at least one outer surface and a control subsystem adapted to supply power to the heating subsystem to maintain the outer surfaces at a temperature above a dew point temperature associated with a coldest temperature location on the at least one outer surface.

The present invention provides a method of reducing and/or eliminating sweat or condensation on outer surfaces of a cooled container, where the method includes the step of cooling a container including at least one outer surface. After being cooled, a lowest temperature or critical temperature on the at least one outer surface is determined, without supplying power to a heating subsystem, to determine a critical temperature. After the critical temperature is determined, a temperature of a location on or associated with the at least one outer surface is measured to obtain a measured temperature. Once the critical temperature and measured temperature is determined, a temperature difference between the measured temperature and the critical temperature is calculated to obtain a diversity factor. Before, after or concurrently with measuring the measured temperature, a relative humidity of the container's surroundings and a surrounding air temperature are measured. From the relative humidity and the surrounding air temperature, a dewpoint is calculated. Once the diversity factor is determined, power is supplied to the heating subsystem to maintain the at least one outer surface at a temperature greater than or equal to the diversity factor minus the calculated dewpoint temperature derived from the surrounding air temperature and the relative humidity. Adjusting the power supplied to heating subsystem in response to changes in the measured temperature and the relative humidity. It should be recognized that if the measured temperature happens to be at a position at or near the location having the coldest temperature, the diversity factor is zero.

The present invention also provides a method of calibrating a cooling container, where the method includes the step of cooling a container include at least one outer surface. After being cooled, a lowest temperature or critical temperature on the at least one outer surface is determined, without supplying power to a heating subsystem, to determine a critical temperature. After the critical temperature is determined, a temperature of a location on or associated with the at least one outer surface is measured to obtain a measured temperature. Once the relative humidity, critical temperature and measured temperature is determined, a temperature difference between the measured temperature and the critical temperature is calculated to obtain a diversity factor. Before, after or concurrently with measuring the measured temperature, a relative humidity of the container's surroundings is measured. Once the diversity factor is determined, a power profile is calculated to maintain the at least one outer surface at a temperature greater than or equal to the diversity factor minus a calculated dewpoint temperature derived from measured temperatures and relative humidities.

DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the following detailed description together with the appended illustrative drawings in which like elements are numbered the same:

FIG. 1A depicts a preferred embodiment of this invention;

FIG. 1B depicts another preferred embodiment of this invention;

FIG. 1C depicts another preferred embodiment of this invention; and

FIG. 2 depicts a preferred controller of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have found that an anti-sweat heater control system can be incorporated into a refrigeration circuit of a refrigerated container or plurality of refrigerated containers, where the control system includes a plurality of heaters, one for each door frame or outer surface for which anti-sweat protection is desired, a temperature sensor located at a location on or associated with at least one door frame or outer surface of the refrigerated container and a zone relative humidity sensor, where the heaters are turned on whenever the temperature sensor reading is below a diversity factor corrected set point temperature and where the diversity factor is a difference between the temperature sensor reading and a critical temperature. The benefits of using a “diversity” factor verses a fixed set point are that: (1) the temperature sensor can be located where it is convenient rather than having to locate the sensor at the critical location, which may not be in a particularly convenient location; (2) the system can be easily adjusted in the field to compensate for variations in surroundings, construction or the like; and (3) the system will save money over a fixed set point approach having a temperature sensor not optimally positioned and will work as well as a unit with a temperature sensor located at the critical location.

The present invention broadly relates to a refrigeration system including a container having at least one door frame or outer surface for which anti-sweat protection is desired, where the frame or surface optionally may include a transparent member exposed to the container's surrounding and a frame/member heater adapted to maintain the member in a substantially sweat-free condition. The system also includes a refrigeration unit for cooling an interior of the container, a temperature sensor affixed (permanently or temporarily) to a location associated with or on the member, the frame or the outer surface and a humidity and surrounding air temperature sensor adapted to determine a relative humidity of the container's surroundings. The system also includes a control system that controls power to the heaters so that a temperature of the member, frame or outer surface is equal to or above a set point temperature, where the set point temperature is a dewpoint temperature plus a diversity factor, where the dewpoint is derived from the surrounding air temperature and the measured relative humidity. The diversity factor is a difference between the temperature at the measured location minus a temperature, the critical temperature, of a coldest location, the most critical spot, on the member, frame or outer surface, when no power is supplied to the heaters and the container is in a stable cooled condition. Thus, by supplying sufficient power to the heater, the member, frame or outer surface can be maintained at a temperature above the dewpoint temperature of the most critical spot, i.e., above the critical temperature. Again, the measured temperature location may be at or near the critical spot causing the diversity factor to be zero.

The present invention broadly relates to a refrigerated display case including at least one door having a transparent member exposed to its surroundings and a frame/member heater adapted to maintain the member in a substantially sweat-free condition. The case also includes a refrigeration unit for cooling an interior of the case, a temperature sensor affixed (permanently or temporarily—removably) to a position associated with or on the door or member and a sensor adapted to determine a relative humidity of the surroundings and a surrounding air temperature. The case also includes a controller for supplying power to the heater sufficient to maintain the door and/or the member in a substantially sweat-free condition. The substantially sweat-free condition is obtained by maintaining the door and/or member at a temperature that is equal to or above a set point temperature, where the set point temperature is a dewpoint temperature plus a diversity factor, where the dewpoint is derived from a measured temperature and a measured relative humidity. The diversity factor is a difference between a temperature at a measured location minus a temperature, a critical temperature, of a coldest location, the most critical spot, on the member and/or door, when no power is supplied to the heaters when the container is in a stable cooled condition. Thus, by supplying sufficient power to the heater, the member, frame or outer surface can be maintained at a temperature above the dewpoint temperature of the most critical spot.

The present invention broadly relates to a method for controlling sweat on a refrigeration system including the step of providing a container having at least one outer surface such as a door including a transparent member, where the surface and member are exposed to its surroundings, a surface heater adapted to maintain the surface and/or member in a substantially sweat-free condition, a refrigeration unit for cooling an interior of the container, a frame temperature sensor affixed (permanently or temporarily—removably) to a location associated with or on the surface or member and an environment sensor adapted to determine a relative humidity of its surroundings and a surrounding air temperature. The method also includes the step of determining a diversity factor from a temperature measured by the sensor and a temperature, a critical temperature, of a critical spot or location on the outer surface or member (the coldest temperature location on the outer surface or member) when the container is in a stable cooled condition. The method also includes the step of obtaining a frame temperature from the frame sensor and a relative humidity and a surrounding air temperature from the environment sensor. The method also includes the step of calculating a relative set point temperature derived from the surrounding air temperature and the relative humidity and corrected by the diversity factor. The method also includes the step of supplying power to the heater sufficient to maintain the temperature of the outer surface and/or member at a temperature above or equal to the relative set point temperature.

Referring now to FIG. 1A, a preferred embodiment of this invention, generally 100, is shown to include a display case 102 having two doors 104. The doors 104 include door handles 106. The case 102 also includes a door frame 108. The case 102 includes a cooling unit 110, a frame temperature sensor 112, a humidity and surrounding air temperature sensor 114 (the sensor includes a humidity sensing component and a temperature sensing component), frame and door heaters 116 and a control unit 118. Although the sensor 114 is shown associated with the frame 108, the sensor 114 can be located remote from the case 102, provided, of course, that it is located in the same general location as the case 102, such as in the same room. Moreover, the sensor 114 can be replace be two sensor housed separately, a humidity sensor and a surrounding air temperature sensor. The control unit 118 is connected to the cooling unit 110, the heaters 116, the frame temperature sensor 112 and the sensor 114 via wires 120. The control unit 118 is adapted to control the cooling unit 110 and the frame and door heaters 116 based on input data from the frame temperature sensor 112 and the sensor 114. The data are processed with respect to a diversity factor so that the heaters 116 are set to a sensor temperature corrected by the diversity factor maintaining all exposed parts of the frame 108 and the doors 104 above the current dew point as determined from the sensor 114 (from the relative humidity and surrounding air temperature). In this embodiment, the frame temperature sensor 112 is located at a frame bottom position 122.

Referring now to FIG. 1B, a preferred embodiment of this invention, generally 100, is shown to include a display case 102 having two doors 104. The doors 104 include door handles 106. The case 102 also includes a door frame 108. The case 102 includes a cooling unit 110, a frame temperature sensor 112, a humidity and surrounding air temperature sensor 114 (the sensor includes a humidity sensing component and a temperature sensing component), frame and door heaters 116 and a control unit 118. The control unit 118 is connected to the cooling unit 110, the heaters 116, the frame temperature sensor 112 and the sensor 114 via wires 120. The control unit 118 is adapted to control the cooling unit 110 and the frame and door heaters 116 based on input data from the frame temperature sensor 112 and the sensor 114. The data are processed with respect to a diversity factor so that the heaters 116 are set to a sensor temperature corrected by the diversity factor maintaining all exposed parts of the frame 108 and the doors 104 above the current dew point as determined from the temperature sensor reading and the humidity. In this embodiment, the frame temperature sensor 112 is located at a frame side position 124.

Referring now to FIG. 1C, a preferred embodiment of this invention, generally 100, is shown to include a display case 102 having two doors 104. The doors 104 include door handles 106. The case 102 also includes a door frame 108. The case 102 includes a cooling unit 110, a frame temperature sensor 112, a humidity and surrounding air temperature sensor 114 (the sensor includes a humidity sensing component and a temperature sensing component), frame and door heaters 116 and a control unit 118. The control unit 118 is connected to the cooling unit 110, the heaters 116, the frame temperature sensor 112 and the sensor 114 via wires 120. The control unit 118 is adapted to control the cooling unit 110 and the frame and door heaters 116 based on input data from the frame temperature sensor 112 and the sensor 114. The data are processed with respect to a diversity factor so that the heaters 116 are set to a sensor temperature corrected by the diversity factor maintaining all exposed parts of the frame 108 and the doors 104 above the current dew point as determined from the temperature sensor reading and the humidity. In this embodiment, the frame temperature sensor 112 is located at a side door position 126.

Referring now to FIG. 2, a schematic of a preferred embodiment of a heater control system 118 of this invention is shown. The unit 118 comprises a board 128 having a humidity sensor input 130 and a surrounding air temperature sensor input 132. The board 128 also includes four frame temperature inputs 134. One of the four frame temperature input 134 a is shown connected to a frame temperature sensor 112. The remaining inputs 134 b-d are not shown connected to corresponding frame temperature sensors 112. Thus, the control unit 118 can independently control a plurality of cases 102 or other cooled containers. The board 128 also includes an AC power input 136, a transformer 138, a control chip 140 and three relays 142, each shown connected to a corresponding frame heater 116; again, demonstrating that a single control unit 118 can control a plurality of heaters to allow the container's outer surfaces to remain sweat free. The board 128 also includes various electronic components such as resistors, capacitors, inductors or the like that permit the board to act as a control unit. Although the invention describes the control unit 118 as controlling both the cooling unit and the heaters, the apparatus of this invention can also include a separate controller for the cooling unit that permits the cooling unit to keep the interior of the container at or below a desired temperature, while a second control unit controls the heaters to eliminate sweat using data from the temperature sensors and the humidity sensor.

The board 128 can include the following components in a number of configurations:

-   -   XE-CM: Serial and Ethernet Communications Manager     -   XE-IO: Eight Input/Output Controller     -   XE-PWR3 Power Supply; 115/230 VAC in; 3 Relays     -   XE-TERM Plug-On Input Terminal Board for XE-IO     -   Mounting Track 12″ Long     -   Mounting Track 18″ Long     -   TM-1 Temperature Sensor (cylindrical)     -   TM-4 Temperature Sensor (square)     -   DP2 Dewpoint Sensor     -   CRX1 Relay Board (1 addressable relay)     -   CRX4 Relay Board (4 addressable relays)     -   Cable, accessory 3″ long     -   Cable accessory 24″ long     -   Cable accessory 6′ Long

Experimental Section

In a Winn-Dixie store in Orlando, an anti-sweat apparatus was installed to determine an improved method for reducing or eliminating sweat on exposed surfaces of a cooled or refrigerated display case. The approach was to monitor the temperature of a spot on one door frame for each refrigeration circuit and control all the anti-sweat heaters associated with the fixtures included in that refrigeration circuit a few degrees above the calculated dew-point (based on the measured zone temperature and the measured zone relative humidity).

During the test, it was discovered that the temperature sensor as not located at the most critical location, and, therefore, sweat still accumulated on surfaces of the cooled display case. The monitored location was clear of moisture, but other parts of the frame were colder and had condensation. At that time, instead of moving the temperature sensor to the most critical location, i.e., the location that has the lowest temperature on the exposed surface, a new approach was developed for maintaining the selected frame temperature a fixed value above a calculated dew-point, by adjusting a value of the set point temperature based on the calculated dew-point. If the dew-point was near the design level of 55° F. or 60° F., the selected frame temperature was maintained a few degrees above the calculated dewpoint. However, as the zone's calculated dewpoint lowered, the setpoint was increased proportional to a “diversity” factor. The diversity factor is arrived at by measuring how much colder the most critical spot is verses the spot selected for mounting the frame temperature sensor. In the test site, it was found that the door mullion temperature would go as low as 20° F. when the heaters were turned off for an extended time, but the frame temperature sensor would only go as low as 40° F.

Tests were continued at the Winn-Dixie store and the new system was installed on seven more refrigeration circuits. The diversity factor is measured once and then used for all other installations of similar design. After testing and development, the final method included the steps of: (1) installing a frame temperature sensor at a sensor frame location of a refrigerated fixture including a cooling system, anti-sweat frame heaters and an exposed surface for which the anti-sweat heating is required; (2) turning off all power to the anti-sweat frame heaters; (3) cooling the fixture for a time sufficient for the frame and the exposed surface to achieve their design temperatures; (4) determining a coldest surface temperature on the frame or the exposed surface; (5) measuring a coldest frame temperature; (6) measuring a frame sensor temperature; and (7) subtracting the coldest temperature from the frame sensor temperature to yield a diversity factor. For example, if the coldest frame temperature was measured to be 22° F. and the frame sensor temperature was measured to be 36° F. Then the diversity factor is 36−22° F. or 14° F. A conventional controller would control the frame at a temperature as though a dewpoint was at 36° F., while the controller of this invention would be actually control the frame as if the dewpoint temperature of the frame sensor location were 22° F. instead of 36° F. If the diversity factor is not applied to the frame sensor temperature, then portions of the frame or exposed surface would potentially be below the dewpoint temperature and would sweat.

All references cited herein are incorporated herein by reference. While this invention has been described fully and completely, it should be understood that, within the scope of the appended claims, from reading this description, those of skill in the art may appreciate changes and modifications that may be made which do not depart from the scope and spirit of the invention. 

1. A refrigeration system comprising: a container subsystem including: an interior; at least one exposed surface, a container temperature sensor; a surrounding air temperature sensor; and a humidity sensor; a container cooling subsystem adapted to cool the interior of the container to a desired interior temperature; an anti-sweat heater subsystem; and a control subsystem adapted to control the cooling subsystem and the anti-sweat heaters, where the anti-sweat heater subsystem is designed to maintain all exposed outer surfaces at a temperature above a dewpoint temperature of the container's surroundings or to maintain all locations on the exposed surfaces in a substantially sweat-free condition and where the controller controls the anti-sweat heater subsystem at a frame sensor temperature plus a diversity factor, where the diversity factor is a difference between the container sensor temperature and a coldest outer surface temperature.
 2. The system of claim 1, further comprising a door frame and at least one door.
 3. The system of claim 1, wherein the at least one door includes a transparent member exposed to the container's surrounding.
 4. A refrigerated display apparatus comprising a case including a frame having a door, a sensor system adapted to measure a surrounding air temperature, a working temperature and a surrounding air humidity, a cooling system designed to cool an interior of the case to a desired reduced temperature, a heater system adapted heat the frame and door and a controller adapted to supply power to the cooling system sufficient to cool the interior of the case, the heater system sufficient to maintain all locations of the frame and the door at a temperature above a dewpoint temperature of the container's surroundings or to maintain all locations on the frame and the door in a substantially sweat-free condition, to the sensor system and to receive data from the sensor corresponding to the surrounding air temperature, the working temperature and the surrounding air humidity, where the controller determines the dewpoint temperature from the surrounding air temperature and humidity and supplies power to the heater system corresponding to the dewpoint temperature plus a diversity factor corresponding to a temperature difference between the working temperature and a coldest door or frame temperature.
 5. The system of claim 1, where the door includes a transparent member.
 6. A method for controlling sweat on a refrigerated container comprising the steps of: providing the container comprising one surface or a plurality of outer surfaces exposed to their surrounding, a heater system, a sensor system including a surrounding air humidity sensor, a surrounding air temperature sensor and a frame temperature sensor, a cooling system and a control system; measuring a surrounding air humidity, a surrounding air temperature, and a frame temperature; determining a diversity factor corresponding to a temperature different between the frame temperature and a coldest temperature corresponding to a coldest temperature location on the outer surfaces when the container is in a stable cooled condition; calculating a surrounding's dewpoint temperature from the surrounding air temperature and humidity; calculating a relative set point temperature derived from the dewpoint temperature corrected by the diversity factor; and supplying power to the heater system sufficient to maintain the temperature of the outer surfaces at a temperature above or equal to the relative set point temperature.
 7. A method for determining a diversity factor comprising the steps of: providing a refrigerated container comprising one surface or a plurality of outer surfaces exposed to their surrounding, a heater system, a sensor system including a surrounding air humidity sensor, a surrounding air temperature sensor and a frame temperature sensor, a cooling system and a control system; turning off power to the heater system; cooling the container for a time sufficient for the container to achieve a sable cooled temperature, determining a coldest outer surface temperature; subtracting the coldest outer surface temperature from a frame sensor temperature to produce a diversity factor. 